A typical grid-tied photovoltaic (PV) installation 10 is shown in FIG. 1, built with arrays of PV modules 12-14 that are series-connected to a string inverter 16. The output from the string inverter 16 is connected to an ac grid 18 to which ac power is supplied.
An emerging system architecture 20, is shown in FIG. 2. The emerging system architecture 20 supplements the string-inverter paradigm, using a plurality of dc-dc converters 22-24 dedicated to respective individual PV modules 12-14. The dc-dc converters of the type used herein may be operated with microcontrollers 22-24, and are sometimes referred to herein as “dc-dc microconverters.” The basic power-converter circuits may have, for example, buck, boost, buck-boost, Ćuk converter current control capabilities.
The dc-dc microconverters 22-24 in the architecture 20 of FIG. 2 provide a number of advantages. In particular, conventional systems are known to underperform if any of the individual PV modules 12-14 in a series string are partially shaded, for example, due to cloud cover or shadowing. The systems may also underperform if they are non-uniformly illuminated, for example, due to different roof angles in residential settings. The systems may also underperform if they are mismatched, for example, due to aging or manufacturing differences of the PV panels. However, the use of dc-dc microconverters enable these problems to be addressed by enabling a maximum power point tracking (MPPT) algorithm to be implemented at the individual PV module level so that underperforming modules do not constrain the whole PV string or array.
More particularly, when light striking a PV module is not uniform, multiple peaks or maxima may exist in power versus output current or power versus output voltage. These local maxima confuse perturb-and-observe (P&O) algorithms that simply increment or decrement a control variable (output current or output voltage) to increase power.
In order to identify the power maxima the entire range of possible voltages must be scanned, requiring large amounts of time. Additionally, once a power maximum is determined from the identified power maxima in order to start a P&O algorithm, generally if shading or other conditions changes, the same parameters continue to be used, thereby requiring very large adjustment times to accommodate the changed conditions.
In addition, in the past, MPPT algorithms do not provide for continuous maximum power point switching between power modes, such as between buck and boost modes, or vice versa. This results in reduced efficiency during mode switching, which requires relatively large components, such as electrolytic capacitors, or the like, for compensation, which, in turn, results in a relatively large form factor of the PV microconverters in the PV system.
What is needed is a method, system, and circuit for determining the MPP, without scanning the entire power range over which the PV array is operated to enable a P&O algorithm to be started in a manner that avoids the P&O algorithm from being confused by a plurality of local power maxima. What also is needed is a method for providing continuous maximum power switching between power modes.